A plasma supply device is a plasma current supply that supplies plasma processes with electric power. The plasma supply device operates at a basic frequency that, when used as a plasma power supply, should only deviate slightly from a theoretical value. Typical basic frequencies in the radio frequency (RF) spectrum are, for example, 3.39 MHz, 13.56 MHz, 27 MHz, 40 MHz and 62 MHz. An inverter, which includes two switching elements, generates from a DC signal of a DC power supply an alternating signal that changes its sign or its amplitude periodically at the rate of the basic frequency. For this purpose, the switching elements are switched backwards and forwards between a conducting and a non-conducting state at the rate of the basic frequency. An output network generates from the alternating signal generated by the inverter a sinusoidal output signal at essentially the predetermined basic frequency.
A plasma is a special aggregate condition that is produced from a gas. Every gas consists in principle of atoms and/or molecules. In the case of a plasma, the gas is largely ionized, which means that the atoms and/or molecules are split into positive and negative charge carriers, i.e. into ions and electrons, due to the supply of energy. A plasma is suitable for processing workpieces because the electrically charged particles are chemically highly reactive and can also be influenced by electrical fields. The charged particles can be accelerated by means of an electrical field on a workpiece, where they can release individual atoms from the workpiece on collision. The released atoms can be removed by gas flow (etching) or coated on other workpieces (production of thin films). A plasma can be used to deposit extremely thin layers, for example, in the region of few atom layers. Typical applications for plasma processing are semiconductor technology (coating, etching, etc.), flat screens (similar to semiconductor technology), solar cells (similar to semiconductor technology), architectural glass coating (heat protection, dazzling protection, etc.), storage media (CD, DVD, hard discs), decorative coatings (colored glasses, etc.) and tool hardening. These applications impose high demands in terms of accuracy and process stability.
Furthermore, a plasma can also be used to excite lasers, particularly gas lasers.
To generate a plasma from a gas, energy is supplied to the gas. Energy can be supplied in different ways, for example, as light, heat or electrical energy. Moreover, a plasma can be ignited with the electrical energy. A plasma for processing workpieces is typically ignited in a plasma chamber, for which purpose an inert gas, e.g. argon, is generally filled into the plasma chamber at low pressure. The gas is exposed to an electrical field that is produced by electrodes and/or antennae.
A plasma is generated or is ignited when several conditions are met. A small number of free charge carriers must be present, and in most cases use is made of the free electrons that are always present to a small extent. The free charge carriers are accelerated so much by the electrical field that they create additional electrons when colliding with atoms or molecules of the inert gas, thus producing positively charged ions and additional electrons. The additional charge carriers are again accelerated and on collision produce additional ions and electrons. An avalanche effect is created. The natural recombination counteracts the constant generation of ions and electrons, i.e. electrons are attracted by ions and recombine to form electrically neutral atoms and/or molecules. Therefore energy is constantly supplied to an ignited plasma in order to maintain the plasma.